In the agricultural industry, considerable research has been done and is ongoing to determine optimum plant spacing for various crops to provide maximum yields. This desired spacing in turn gives a desired plant population of plants per unit of area, such as a square foot or acre. In order to translate this into a usable form for the farmer, the average number of seeds in a unit of weight, such as pounds, is determined for an average sample of representative seeds. The number of desired plants per acre is then translated into a seeding rate of pounds per acre.
Seed sizes and weights as well as desired plant populations vary dramatically between different crops. A typical corn variety has large sized seeds and may have about 1200 seeds per pound, while a typical wheat variety has smaller medium sized seeds and may have about 12,000 seeds per pound, and a typical canola has very much smaller sized seeds and may have about 120,000 seeds per pound.
Similarly desired plant populations also vary dramatically. A typical population for corn may be about 0.5 plants per square foot (22,000 plants per acre), for canola about 5 plants per square foot (220,000 plants per acre), and for wheat about 25 plants per square foot (1,100,000 plants per acre). Canola seed can cost up to $10 per pound, and so current research is showing that the traditional plant population of 10 plants per square foot can be reduced to 5 per square foot if fairly evenly distributed.
Ideally these plants are spaced equally in all directions in a grid like arrangement however in practice a given seeder has a fixed spacing between the furrow openers and so the spacing between the rows of plants is fixed for any particular seeder, typically for a modern minimum tillage air seeder these row spacings are 10-12 inches. With the spacing between rows fixed, altering the seeding rate alters the spacing between seeds in a row.
With a wheat crop on a 12 inch row spacing, there should be about 25 seeds per foot of row, and the distance between seeds is then less than ½ inch. With canola there should be about 5 seeds per foot of row, and the distance between seeds is 2½ inches.
With corn there should be about 0.5 seeds per foot of row, and the distance between seeds 24 inches. For maximum yields however, corn is seeded at a wider row spacing, typically 30 inches or more, with a row type planter. Corn is sometimes seeded with a 12 inch spacing air seeder by only using every other furrow opener so the row spacing is 24 inches and there are thus about 1.0 seeds per foot of row.
Thus it can be seen that to get an even plant spacing, the spacing is much more critical for corn than for either wheat or canola, while the spacing of canola seeds is more critical than for wheat. To achieve the desired even spacing, singulating metering devices are commonly used for planting corn. These typically comprise pockets or recesses on a rotating disc and take the seeds from a seed tank one at a time and drop them into a furrow.
In contrast, seed metering devices for seeding wheat and canola typically work on more of a bulk principal. One typical type of seed metering device includes a rotating feed roller with recesses that can be in the form of grooves extending longitudinally along the length of the outer surface of roller parallel to the axis of rotation, or in the form of notches or pockets or the like spaced evenly along the outer surface of the roller. The feed roller is typically mounted below the feed opening at the bottom of a seed tank and configured such that the intake side of the roller is inside the feed opening exposed to the seeds in the tank and the outer output side is above the furrow in a conventional seeder, or in an air seeder is above the air stream that carries the seeds to the furrow openers to be deposited in the ground. As the roller rotates, seeds fill the recesses and are carried from the seed tank and dropped into the furrow in a conventional seeder, or are fed into the air stream of an air seeder.
The number of seeds dispensed is proportional to the rotational speed of the feed roller or auger, and the rate of metering seeds is adjusted by varying the rotational speed. The rotational speed is also coordinated with ground speed so that the same amount of seed is dispensed for each foot of distance travelled.
As a feed roller rotates, the seed is dispensed essentially in pulses. The recesses spill their contents into the furrow or air stream as they rotate, with one emptying before the next starts to spill. With wheat this is not particularly problematic, as the seed spacing is not so critical, and in addition the volumes of seed required to be dispensed for a typical seeding rate require a rotational speed that is relatively high so that the pulses blend together and the result is a fairly constant flow of wheat seeds that is satisfactory for seeding wheat.
With canola however, the number of seeds that are contained in any particular recess is about 10 times the number of wheat seeds that are contained in. the same groove or recess. In addition it is desired only to have 5 canola seeds per foot instead of 25 wheat seeds, so a feed roller rotating at a speed suitable to dispense the desired rate of wheat seeds will dispense canola seeds at about 50 times the desired rate.
For seeding canola then, the speed of rotation must be reduced to 1/50 the rotational speed for wheat, however reducing the speed that much becomes problematic as the pulsing nature of the flow of canola seeds is accentuated. The flow of canola seeds into the air stream is very uneven, and although the correct number of seeds per foot of row will be dispensed, the seeds will be deposited in the furrow in clumps instead of evenly spaced. In order to address this problem, different feed rollers are often used for wheat and canola. The canola feed rollers will have shallower and smaller recesses such that less seed is dispensed with each revolution, and the rotational speed can be increased and the pulsing effect reduced. The rotational speed of these canola feed rollers however is still very slow, in the order of 1 revolution per minute. At these slow speeds, and given the nature of a canola seed being very small and round, the movement and vibration caused by the seeder travelling along a field causes the canola seed to shake out of the recesses, accentuating the uneven flow.